1. Field of the Invention
The present invention relates to a paper bailing apparatus for a printer and, more particularly, to a mechanism of driving a bail roller for holding paper against a platen.
2. Description of the Related Art
In a conventional printer of this type, printing paper is brought into contact with the peripheral surface of a platen, and printing on the paper is carried out by a printing head held by a carriage which reciprocates in the axial direction of the platen.
At this time, the paper is generally held against the platen by a bail roller so that the paper is in close contact with the platen when printing is carried out.
Such a bail roller is generally rotatably supported at one end of a bail lever, and after the paper is fed to the platen, the bail roller which has been held apart from the platen is moved to the bailing position.
Such a bail roller must be positioned securely at the bailing position after the paper is mounted on the platen.
At the time of inserting paper, however, the bail roller is rather a hindrance, and if the paper is inserted carelessly when the bail roller is in the bailing position, a jam will be caused.
Therefore, when roll paper or sprocket paper is inserted, the bail roller is manually moved to the release position by the user, and after the paper is securely mounted on the platen, the bail roller is manually restored to the bailing position by the user.
Recent automatic printers, however, are required to automatically drive the bail roller, and some automatic bail roller driving devices have hitherto been proposed.
Conventional automatic bail roller driving mechanisms generally have a solenoid or a motor exclusively for driving a bail lever, and when a sensor detects the insertion of paper, the solenoid or the motor is driven in accordance with a predetermined control program for moving the bail roller.
Such a conventional device requires various driving mechanisms merely for moving the bail roller, so that the size of the device is inconveniently increased, resulting in an increase in cost of the printer. An improved automatic bail roller driving device which is applicable to a recent small-sized, light-weight and inexpensive printer has therefore been demanded.
Another automatic bail roller driving mechanism conventionally proposed is a device which utilizes the carriage driving force to also drive the bail roller without providing any plunger or motor exclusively for driving the bail roller.
An example of such a mechanism is disclosed in Japanese Patent Laid-Open No. Sho 63-49465. In this mechanism, the paper feed roller rotating force is transmitted to a rotary cam through a clutch so as to rock a bail lever which is connected to the cam. The operation of connecting or disconnecting the clutch is carried out by moving the carriage beyond the printing range.
This mechanism is advantageous in that the bail roller is automatically driven without using any plunger or motor exclusively therefor, but suffers from the following problems.
(1) Even when the user manually rotates the bail lever so as to release it, since the bail lever is constantly biased toward the bailing position by a spring, it is impossible to hold the bail lever at the release position.
Therefore, when the user manually sets paper, the user must hold the bail lever so as to prevent the bail lever from being restored to the bailing position. The operability is thus poor.
(2) When the bail lever is at the release position, in other words, while the cam is holding the bail lever, the bail lever cannot be manually restored to the bailing position. If the bail lever is restored to the bailing position by force, the cam mechanism or the bail lever will be broken.
(3) Since an arm which engages the carriage and rotates in accordance with the movement of the carriage is not provided with any bending mechanism or buffer mechanism, if the amount of movement of the carriage after it engages the arm becomes too large by mistake, a part will be broken.
The carriage often moves until it hits against the frame when the control of the position of the carriage is lost, for example, when the carriage is manually moved by the user for the purpose of changing ribbons while the power source is off or when the carriage motor is out of order. In such a case, inconvenience such as breakage of the carriage, deformation of the arm and breakage of a tooth of the gear is caused.
(4) It is necessary to recognize the stopping position of the cam and initialize the cam at a predetermined position when the power source is turned on.
For this purpose, not only are the laborious operations of the carriage, the paper feed roller (platen) and the bail lever required whenever the power source is turned on, but also when the power source is turned on when paper is already mounted, the driving operation of the paper feed roller for initialization deviates the position of the paper which has been set at the right position or obstructs the normal paper feed.
Furthermore, this mechanism requires a detecting means such as an encoder for detecting the rotational position of the cam in order to control the cam, which leads to an increase in cost.
As described above, although this automatic bail roller driving mechanism dispenses with a driving source exclusively for its use, the manual operability of the bail roller by the user is sacrificed. In addition, since even a small operational error is apt to lead to breakage, this mechanism cannot be said to be satisfactory.
An automatic bail roller driving mechanism having a simpler structure while maintaining the manual operability is disclosed in Japanese Patent Laid-Open No. Sho 63-42879.
In this mechanism, the bail lever is manually driven and is held at the release position, and when the carriage is moved beyond the printing range, the carriage triggers the releasing mechanism for moving the bail lever from the release position so as to return to the bailing position.
This mechanism is advantageous in that printing is efficiently possible from the upper end of the paper, but suffers from the following problems.
Since the carriage is constantly out of the printing range when the bail lever is restored to the bailing position, the paper is horizontally asymmetrically slackened when the bail roller is pressed against the platen and at the subsequent paper feeding stage, the paper is fed obliquely, thereby causing what is called skew, in other words, printing in a skewed line.
The known skew phenomenon is generally prevented by temporarily moving the carriage to the vicinity of the center position (this operation is generally called centering) before restoring the bail lever to the bailing position, so that the bail roller is pressed against the platen while the paper is held substantially in a horizontally symmetrical position by the printing head, the guide provided on the carriage and the like.
In this automatic bailing roller driving mechanism, however, it is impossible to center the carriage before restoring the bail lever to the bailing position. Therefore, in spite of the simple structure, this mechanism possesses a major defect, namely, skew of paper.